So-called PSA plants, that is to say plants for implementing the PSA cycles mentioned above, are enjoying increasing success, in particular in the fields of purifying hydrogen, treating natural gas, separating the gases in air, recovering solvents, and fractionating synthesis gas.
This success is leading PSA plant manufacturers to construct plants which, although they perform better, are also becoming increasingly complex.
This can be seen from the fact that the plants comprise a plurality of adsorbers with which a large number of valves are associated.
Thus, for example, a PSA plant described in document U.S. Pat. No. 4,834,780 comprises six adsorbers and thirty-seven valves, and another described in document U.S. Pat. No. 4,475,929 comprises ten adsorbers and sixty-seven valves.
Given that proper running of the cycle depends on the sequences of opening and closing these valves which make it possible to obtain the intended pressure cycle, it can therefore readily be seen that malfunction of one of these valves which are operated on each cycle can cause serious problems in the operation of a PSA plant.
By way of example, two cases of malfunction affecting the valves should in particular be highlighted:
1) A first malfunction may consist in mechanical obstruction of the valve, with the result that the valve remains closed in spite of an opening instruction which is applied (or vice versa), or it opens and closes only very slowly.
Such incidents can be detected by end of travel sensors installed in the valves and, where appropriate, connected to timer systems.
Furthermore, given that this type of malfunction generally causes significant imbalances in the pressure cycle of the various adsorbers, such an incident is relatively easy to identify.
2) A second malfunction may result from a sealing defect of a closed valve, which leads to internal leaks either between the adsorbers or between an adsorber and the production line or the residue line.
Unlike mechanical obstruction, leaks in a closed valve which is not leaktight are difficult to detect on a unit which is in operation.
They nevertheless cause a drop in performance of the PSA plant, due to imbalanced operation of the adsorbers or due directly to a loss of production to the residual gas line.
In order to identify the valves which have a sealing defect in the closed position, periodic maintenance shutdowns are conventionally employed to carry out seal tests.
However, this type of inspection has the drawback that it is time-consuming and laborious. Furthermore, in view of the durations of the plant shutdown, these inspections can be carried out only at distant time intervals.
However, given that this loss of production which is caused by a sealing defect of a valve and leads to a few % drop in the extraction efficiency can last several months, the losses may be significant.